1. Field of the Invention
This invention generally relates to methods and apparatus for sensing images of a hemispherical scene and more particularly to methods and apparatus for automatically and omni-directionally sensing depth information for objects within the hemispherical scene.
2. State of the Art
Surveillance, robotics, teleconferencing, and other applications require that an image sensor receive as much information as possible about the environment surrounding it, but the the camera lenses currently available tend to restrict the image sensors' field of view to less than a full hemisphere. In order to photograph or monitor a fully hemispherical field of view, a wide-angle lens must be used or the camera must be panned or tilted across the sector of interest. Both approaches are disadvantageous. Conventional wide-angle lenses, for example, increase a camera's field of view, but still provide field of view that is less than a full hemisphere. Fish-eye lenses, on the other hand, enable the viewing of an entire hemispherical scene, but have short focal lengths and are more bulky and complex than traditional lenses. Panning and/or tilting the camera about its vertical axis enables the sequential viewing of a hemispherical scene, but consumes significant time when rotating through a 180 degree arc. Panning and tilting systems are further disadvantageous in that they are complex and include moving parts that require regular maintenance.
Often a camera's field of view is increased using planar reflective surfaces that are tilted or panned about a central axis to reflect sequential sectors of a hemispherical scene to a camera. Alternatively, curved reflectors are sometimes used. For example, in one method, a substantially paraboloid-shaped reflector is positioned in front of a camera to orthographically reflect radiation from virtually all points of a hemispherical scene to the camera. An orthographic lens on the camera in this system permits a hemispherical scene to be viewed from a single view point but is disadvantageous because it does not provide a method or apparatus for sensing the depth of objects located in the hemispherical scene. Additionally, panning or tilting a planar reflective surface about a central axis is disadvantageous because only one portion of a hemispherical scene at a time is reflected.
Various apparatus and methods are known in the art for detecting the unidirectional depth of an object located in front of a camera. However, no method or apparatus presently exists for omnidirectionally sensing the depth of one or more objects located within a hemispherical scene that surrounds the camera.